The invention relates to a tire with radial carcass reinforcement anchored in each bead to at least one bead wire, and comprising a crown reinforcement formed by at least two so-called working plies, superposed and made of wires or cables which are parallel to each other in each ply and are crossed from one to the next, forming angles of at most 40xc2x0 in absolute value with the circumferential direction of the tire.
More particularly, it relates to a xe2x80x9cheavy vehiclexe2x80x9d -type tire, the ratio of the height above rim H to the maximum axial width S of which is at most 0.60, and which is intended to be fitted on a medium- or high-tonnage vehicle, such as a lorry, bus, trailer, etc.
Tires having crown reinforcements composed firstly of so-called working plies formed of cables oriented at a certain angle with the circumferential direction and secondly of additional plies formed of cables oriented substantially circumferentially are widely known. An example of such tire structure is illustrated and described in U.S. Pat. 3,677,319. In that tire structure the working plies are formed of metallic elements oriented at angles of between 18xc2x0 and 75xc2x0 and the additional plies are formed of textile cables oriented at 0xc2x0. Such a structure makes it possible to reconcile the contradictory qualities which are desired in a tire: for example, road-holding of the vehicle and comfort, wear resistance and rolling resistance. The additional textile plies may be arranged either beneath or on top of or between the plies of metal cables, the width of the textile assembly being between 25 and 75% of the width of the metallic structure.
French Patent 2,419,185 discloses that the type of tire in question, with a low H/S ratio, although it has numerous advantages, also has a certain number of disadvantages, such as poor adhesion of the equatorial zone of the tread or localization of high pressures in the region of the edges of the tread, due to the shortening of the area of contact in the longitudinal direction of the tire. In order to overcome these disadvantages, the cited French patent recommends arranging between the carcass reinforcement and the radially inner working ply, in two zones distant from the equatorial plane, two limiting blocks each formed of two superposed plies of inextensible cables, crossed from one ply to the other, forming opposed angles with the circumferential direction which are at most equal in absolute value to half the smallest angle used in the working plies, and other than 0xc2x0.
The two solutions mentioned above have not been completely satisfactory for improving the endurance of the crown reinforcement of the xe2x80x9cheavy-vehiclexe2x80x9d tire of form ratio of at most 0.6. The problem of endurance presented by the tire of the type in question becomes very complicated: the lack of endurance relates both to the lack of resistance to separation between crown ply ends and to the lack of fatigue strength of the cables of the portion of carcass reinforcement, the meridian profile of which is parallel to the profile of the crown reinforcement, and also to the lack of resistance to separation between the carcass reinforcement and crown reinforcement.
In order to improve said endurance, Application WO 96/20095 describes a crown reinforcement comprising at least two working crown plies of inextensible cables, crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, said plies having widths at least equal to 80% of the maximum axial width S0 of the carcass reinforcement, and characterized in that an axially continuous ply formed of inextensible metal cables forming with the circumferential direction an angle of at least 60xc2x0 and having an axial width at least equal to the axial width of the shortest working crown ply, is arranged firstly between the carcass reinforcement and the working ply radially closest to the axis of rotation, and in that an additional ply formed of metallic elements oriented substantially parallel to the circumferential direction, the axial width of said ply being at least equal to 0.7 S0 , and its modulus of elasticity upon traction being at most equal to the modulus of elasticity of the most extensible working ply, is arranged between the two working crown plies.
Despite the progress made, such a tire of H/S form ratio of at most 0.45 (H being the height of the tire on its mounting rim and S its maximum axial width), intended to replace the set of two dual wheels on a driving axle, has to be improved from the point of view of overall endurance in order to compare favorably with said assembly, and in particular has to be improved in regard to resistance to separation between the carcass reinforcement and crown reinforcement and fatigue strength of the circumferentially oriented cables of the additional ply.
It might have been logical to assume that a modification of structure would have to affect the two regions in which are located the ends of the ply of cables oriented with an angle of at least 60xc2x0 and the ends of the first working ply, and that one possible solution for double problem would have been to increase the width of the ply of circumferential cables. To the great surprise of the inventor, although in fact it proved necessary to increase the number of circumferential cables, this increase does not have to be effected by widening the ply of circumferential cables but by the addition of a second additional ply of circumferential cables of small axial width, arranged radially above the second working ply, that is to say in the extension, respectively on either side of the crown reinforcement, of the radially outermost ply of elastic cables, referred to as protective ply, and by covering the ends of the ply of circumferential cables.
Thus the tire according to the invention, comprising a radial carcass reinforcement surmounted by a crown reinforcement comprising at least two working crown plies of inextensible cables, crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, said plies having widths at least equal to 80% of the maximum axial width S0 of the carcass reinforcement, and, arranged, firstly, between the carcass reinforcement and the working ply radially closest to the axis of rotation, an axially continuous ply formed of inextensible metal cables forming with the circumferential direction an angle of at least 60xc2x0 and the axial width of which is at least equal to the axial width of the shortest working crown ply, and, secondly, arranged between the two working crown plies, a first additional ply formed of metallic elements oriented substantially parallel to the circumferential direction, the axial width of said ply being at least equal to 0.7 S0 , and the modulus of elasticity upon traction of which is at most equal to the modulus of elasticity of the most extensible working ply, is characterized in that there is arranged, radially above the radially outermost working crown ply and on either side of the tire, a second additional ply formed of metallic elements oriented substantially parallel to the circumferential direction, of small width and such that it radially covers the end of the first additional ply of circumferential cables which is arranged between the two working plies.
xe2x80x9cInextensible cablexe2x80x9d is to be understood to mean a cable, for example of steel, which has a relative elongation of less than 0.2% at 10% of the breaking load.
Wires or cables oriented substantially parallel to the circumferential direction are wires or cables which form angles within the range +2.5xc2x0, xe2x88x922.5xc2x0 around 0xc2x0 with said direction.
A modulus of elasticity upon traction of a ply of cables results from the tensile stress, exerted in the direction of the cables, necessary to obtain a given relative elongation ∈, said modulus being a tangent modulus. xe2x80x9cElasticity modulus of the additional ply which is at most equal to the modulus of elasticity of the most extensible working plyxe2x80x9d is to be understood to mean that the tangent modulus of the additional ply, whatever the relative elongation, is at most equal to the tangent modulus of the most extensible working ply whatever the relative elongation, the most extensible ply being the ply which, for each value of tensile stress, has a relative elongation greater than that of the other ply for the same stress.
Advantageously, the modulus of the second additional ply is equal to that of the first additional ply, and will be such that it is small for a relative elongation of between 0% and 0.4%, and at most equal to the greatest modulus of elasticity upon traction of the most extensible working ply, for relative elongations greater than 0.4%.
Within the above framework, the additional plies can be formed of so-called elastic cables, having a curve of tensile stress as a function of the relative elongation which has shallow gradients for the slight elongations and a substantially constant and steep gradient for the greater elongations. The additional plies may also be formed of cables oriented circumferentially and cut so as to form sections of a length very much less than the circumference of the least long ply, but preferably greater than 0.1 times said circumference, the cuts between sections being axially offset relative to each other. Such an embodiment makes it possible to impart to the additional plies a given modulus, whatever it may be, in simple manner.
The second additional ply has its width substantially centered on the end of the first additional ply, such that its own ends are axially distant from the end of the first additional ply by at least 10 mm, a narrow width of ply being a width of between 15 and 50 mm. It is advantageous for said second additional ply axially to extend the radially outermost ply, referred to as protective ply, generally formed of so-called elastic cables forming an angle of the same direction and substantially of the same value as the angle of the subjacent working ply with the circumferential direction, said extension being produced by leaving only a gap of small width ( less than 5 mm) between the axially inner end of the additional ply and the axially outer end of the ply of elastic cables.
The characteristics and advantages of the invention will be better understood with the aid of the following description which relates to the drawings, which illustrate examples of embodiment in non-limitative manner.